The present invention relates to a photoelectric smoke sensor in which light emitting units and photo sensing units are arranged to face with each other with a constant distance apart from each other and the occurrence of fire is detected on the basis of attenuation of the light due to the smoke flowed into the area between the light emitting and photo sensing units installed and, more particularly, to a photoelectric smoke sensor in which a variation in photo sensing signal due to a fouling of the optical system is corrected.
Hitherto, in photoelectric smoke sensors in which light emitting units and photo sensing units are arranged to face with each other, with an elapse of a long use time, the dust is deposited on the windows of the light emitting and photo sensing units, so that a level of the photo sensing signal decreases and if the photo sensing signal level decreased due to the dust becomes a level below a threshold value at which it is determined that the fire occurred, an error fire signal will have been outputted. Therefore, it is necessary to perform the cleaning work to remove the dust at every constant period.
However, since the cleaning work for removal of the dust is troublesome, an apparatus in which the photo sensing signal is automatically corrected in accordance with a degree of deposition of the dust has been considered. For example, a photoelectric smoke sensor disclosed in U.S. Pat. Ser. No. 4,317,113 has been known.
In this photoelectric smoke sensor, a gain of an operational amplifier for amplifying a photo sensing signal is varied step by step in accordance with a degree of attenuation of the signal due to the deposition of the dust, and when the photo sensing signal attenuates due to the deposition of the dust, the amplification gain is increased by an amount as much as a degree of attenuation, thereby making it possible to obtain the photo sensing signal in the same condition as that whereby no dust is deposited.
However, the fouling correction by means of the gain control of the operational amplifier has the following problems.